Is
this what you are looking for?
Numrich Gun Parts M&P 15-22
Unfortunately they aren't shipping anything right now due to the C-19 virus lockdown.
You are looking for the pin that holds the firing pin and guide rod post? I think that the one I linked to is the right one, but not sure.
Edit:
Same pin at Midwest Gun Works. They appear to be shipping, but part is sold out right now.
I just did an internet search for M&P 15-22 parts to find both sites.